Two-part cylinder mould for slip casting
The beauty of slip casting is that you can make the forms much faster than you could do otherwise, and you have the security of knowing that every item will be identical – an important factor for some batch production.
You will need: something to use as a model
There are several useful items you can use as a model for your cylinder
- Plastic drainpipe – perfect for two-part cylinder moulds and widely available from builder’s merchants or DIY stores. It’s available in several widths and can easily be cut to sizes of your choice. Useful, also, because it doesn’t need soap sizing. You can cast the pipe itself, or use it to make a plaster master model.
- Old rolling pins – choose a variety with removable handles because they are flat ended.
- Heavy-duty cardboard tubing – from carpet rolls, etc. These will need to be carefully covered with clingfilm to seal the surface before casting.
In addition you will need:
- Clay to build up around the model (the clay used in previous mould-making exercises will be good – to save using fresh clay)
- Boards to use as a cottle (you could use thick clay slabs if you don’t have boards)
Begin by cutting your model to size – in this example, we are using a plastic drainpipe. If using this, make sure the cut is level so that the cylinder will stand vertically. Sand the cut edges to remove burrs of plastic.
To prevent plaster getting inside the pipe when it’s cast, you will need to seal the ends. You can cut discs to size from wood or Perspex or simply fill the pipe with clay and smooth off the ends to flatten.
Draw a line along the length of the model to define each half (you can do this with masking tape if, as in this example, the pipe is black and therefore difficult to mark clearly.
Pencil an arrow to show the actual half-way mark – it can be easy to mistake which side of the tape is correct.
4 and 5
Moulds for casting differ from those we have made in earlier issues of ClayCraft because they need an extension of clay, or another material, to form the opening of the mould through which the slip is poured. This extension is referred to as the ‘spare’. If there were no spare, the rim of the cast would be very uneven
The spare can be made from clay, although other materials can be used as well (see the list of alternatives in the tips box)
Make your spare 3-5mm smaller than the circumference of the opening – this distance will represent the thickness of wall once the form is slip cast.
Fix the spare onto the end of the model, making sure it is central.
Now neaten up the join to remove excess slip and smooth the clay. Take care not to create any undercuts.
Carefully and lightly, draw a continuation of the half way division across the shoulder of the model and up to the top of the spare with a pin.
TIP: Other materials you can use to make spares
Bottle/jar corks (available in many sizes – look online for suppliers)
Styrofoam – simply cut to the required size.
Wood – MDF
All can usually be stuck on to the end of the model with double-sided tape or similar, but they must be fixed securely.
Now roll a thick slab of clay large enough for the model to lie on with 5 cm spare all the way around except for the end with the spare.
Place the slab on a non-absorbent board then make a central channel for the model to sit on with your rolling pin. This is really just to prevent the model from moving as you work.
To make sure the position is correct, position one of the cottle boards at the end with the spare so that it butts up directly. When cast in plaster this end will then remain open.
Secure the board with a thick coil of clay to stop it from falling over as you get the position right.
Now build up the clay around the model to the half-way mark. Make sure to level it periodically and make sure you don’t create undercuts at the edge of the model itself – especially at the base, where the pipe is filled with clay.
It can be tricky to work around the spare, but this must be divided in the same way – take your time to make sure it is neat and level.
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Build the cottle around the model – seal all the possible escape routes for the plaster with thin coils of clay – including the spare – and secure the boards with string to avoid spillage from the weight of the plaster.
Wipe over the model with a barely damp cloth to remove specks of clay.
You are now ready to cast the first half of the model.
Cast the first half in plaster – the amount required will entirely depend on the size of your model but use the guideline of 1½ lb of plaster (680g) to 1 pint of water (575ml) and scale up the amount accordingly. Click here to see how to mix plaster and make cottles.
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Carefully remove the cottle boards and surform any sharp edges around the plaster.
Now turn the whole thing over and reposition the mould on the board. Carefully lift the base slab clay off the model, which should remain in place in the plaster.
Wipe over the surface of the model to remove traces of clay and remove the tape if you have used it to divide the model.
Make 4-6 ‘natches’ in the plaster by rotating the end of a metal tool like an old knife or melon scoop to form little wells. These will locate the second half of the cast and hold the two parts together properly when casting.
Soap size the surface of the plaster then wipe the size back with a damp sponge. Repeat the process at least twice more and up to seven times to prevent the two halves of the mould sticking together.
Now build the cottle around the model again in the same way as before and again, seal any possible escape routes for plaster with thin coils of clay.
Cast the second half with the same weight of plaster that you used for the first half.
18 and 19
Remove the cottle boards and surform all sharp edges around the plaster.
Very carefully separate the two halves of the mould (see tip for stuck moulds)
Remove the model then carefully wipe away any clay residue from the model with a damp cloth.
Fit the two halves together and put the mould somewhere warm to dry out completely before using – this can take quite a long time depending on the warmth of the drying area – at least a week, but often longer.
The finished image shows the mould held together with sections of inner tube from a bicycle – you will need something similar to hold your mould together when slip casting.
Click here for the slip casting process.
TIP: Unsticking moulds
If your mould will not come apart easily after casting the second half, place a soft cloth over the surface then place a weight or other heavy object on top. Now give the weight a sharp tap with another weight, and the mould should easily pop apart – if not, try again in another position.
For moulds that are really stuck; allow it to dry out somewhat then stand it in a sink with the join side uppermost and pour boiling water over it. As the plaster contracts, the mould should spring apart. Remove the model, then dry the mould with the sections together.